In the advanced information society in recent years, communication devices such as mobile phones, wireless LAN devices, and the like are required to ensure the linearity of a transmission signal over a wide range of power amplification and also operate with low power consumption. In such a communication device, a transmission circuit which outputs a high-precision transmission signal regardless of the bandwidth and operates at high efficiency is employed. A conventional transmission circuit will be described below.
Examples of the conventional transmission circuit include transmission circuits (hereinafter, referred to as a quadrature modulation circuit) which use modulation schemes such as quadrature modulation or the like to generate transmission signals. The quadrature modulation circuit has been widely known, thus the description thereof is omitted. Also, an example of a conventional transmission circuit which operates at higher efficiency than a quadrature modulation circuit is a transmission circuit 500 shown in FIG. 18 (for example, see Non-Patent Literature 1). FIG. 18 is a block diagram showing an example of a configuration of the conventional transmission circuit 500 disclosed in Non-Patent Literature 1. In FIG. 18, the conventional transmission circuit 500 includes a signal generation section 501, a phase modulation section 502, a regulator 503, a power amplifier (PA) 504, and a power supply terminal 505. The power amplifier 504 includes an amplifying transistor.
In the conventional transmission circuit 500, the signal generation section 501 generates an amplitude signal and a phase signal. The amplitude signal is inputted to the regulator 503. The regulator 503 is supplied with a direct-current voltage by the power supply terminal 505. The regulator 503 supplies a voltage in accordance with an inputted amplitude signal to the power amplifier 504. It should be noted that, typically, the regulator 503 supplies a voltage in proportion to the magnitude of the inputted amplitude signal to the power amplifier 504.
On the other hand, the phase signal is inputted to the phase modulation section 502. The phase modulation section 502 phase-modulates the phase signal and outputs a phase-modulated signal. The phase-modulated signal is inputted to the power amplifier 504. The power amplifier 504 amplitude-modulates the phase-modulated signal using the voltage supplied by the regulator 503, and outputs the resultant signal as a modulation signal that is phase-modulated and amplitude-modulated. The modulation signal is outputted as a transmission signal from an output terminal. It should be noted that such the transmission circuit 500 is referred to as a polar modulation circuit.
However, the conventional quadrature modulation circuit has a characteristic that, when the output power is low, the linearity is high but the power efficiency is poor. In addition, the conventional polar modulation circuit has a characteristic that the polar modulation circuit operates at high precision and high efficiency when the magnitude of the output power is greater than a certain magnitude, whereas, when the output power is low, the linearity deteriorates and the power efficiency declines. This is because, when the output power is low, it is difficult to linearly control the power amplifier 504 and also the power amplifier 504 operates outside the saturation region.
Therefore, a transmission circuit which switches the operation mode in accordance with an output power is previously disclosed (for example, see Patent Literature 1). FIG. 19 is a block diagram showing an example of a configuration of a conventional transmission circuit 600 disclosed in Patent Literature 1. In FIG. 19, the conventional transmission circuit 600 uses the quadrature modulation scheme when the output power is low, and uses the polar modulation scheme when the output power is high.
Specifically, the conventional transmission circuit 600 previously has a first threshold value and a second threshold value, provided that the first threshold value > the second threshold value. When the output power is greater than the first threshold value, the conventional transmission circuit 600 connects an output of an AGC amplifier 610 to a base terminal of a power amplifier 611, connects an output of an amplitude modulation circuit 613 to a collector terminal of the power amplifier 611, and uses the polar modulation scheme to amplify an output signal. On the other hand, when the output power is smaller than the first threshold value and equal to or greater than the second threshold value, the conventional transmission circuit 600 connects the output of the AGC amplifier 610 to the base terminal of the power amplifier 611 but the output level is adjusted using the AGC amplifier 610, thereby the output signal is amplified using the linear region of the power amplifier 611. Furthermore, when the output power is smaller than the second threshold value, because, via the power amplifier 611, the quality of the signal may deteriorate due to the effect of distortion in the power amplifier 611, the conventional transmission circuit 600 connects an output of the AGC amplifier 610 to a buffer 612, and outputs the output signal without passing it through the power amplifier 611.
In this manner, the conventional transmission circuit 600 is operable at high precision and high efficiency. When the conventional transmission circuit 600 is used as a mobile wireless device, however, further reduction of the power consumption is required. In particular, most power is consumed by the power amplifier 611 which is a power amplification stage, and thus the reduction of the power consumption in the power amplifier 611 is further required.
Patent Literature 2 discloses a high frequency amplifier which switches a bias voltage by a bias switch along with operation conditions for a power amplifier. In general, for amplification of an analog FM modulation wave, transmission of only a phase change may be necessary, and therefore the use of a class C power amplifier which has high power conversion efficiency is preferred. On the other hand, in the case of a modulation scheme for a digitally modulated wave, for example, a π/4 shift QPSK-modulated wave, which should allow the transmission of both amplitude change and phase change, the use of a class A power amplifier which has excellent linearity is required. Patent Literature 2 discloses a configuration in that a bias voltage to be applied to an amplifying transistor is switched in accordance with a modulation signal, and thereby the operation region of the power amplifier is controlled.
FIG. 20 is a block diagram showing an example of a configuration of a conventional transmission circuit 700 disclosed in Patent Literature 2. In FIG. 20, a modulation wave inputted from an RF input terminal 711 is amplified by a power amplifier 712, and outputted from an antenna 713. The power amplifier 712 includes a grounded-emitter amplifying transistor 718, an input-side match section 717 connected to a base terminal of an amplifying transistor 718, and an output-side match section 719 connected to a collector terminal of the amplifying transistor 718.
A changeover control circuit 715 is connected by a bias switch 720 to the base terminal of the amplifying transistor 718 included in the power amplifier 712. In accordance with a control signal for selecting a modulation wave which is inputted from a control signal input terminal 714, the bias switch 720 connects the base terminal of the amplifying transistor 718 to a first bias circuit 722 or a second bias circuit 723. Specifically, in the case of causing the power amplifier 712 to act as a class C power amplifier, the bias switch 720 connects the base terminal of the amplifying transistor 718 to the first bias circuit 722.
On the other hand, in the case of causing the power amplifier 712 to act as a class A power amplifier, the bias switch 720 connects the base terminal of the amplifying transistor 718 to the second bias circuit 723. In this manner, in the conventional transmission circuit 700, bias circuits 722 and 723 are switched therebetween, depending on a type of modulation wave to be amplified by the power amplifier 712, and thereby the operation region of the power amplifier 712 is converted and the power conversion efficiency is improved.